Saturable reactor keying for radio transmitters



United States Patent SATURABLE REACTOR KEYING FOR RADIO TRANSMITTERS Louis F. Deise, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 5, 1954, Serial No. 402,206

4 Claims. (Cl. 307-106 My invention relates to control systemsfor saturable reactors and is particularly valuable in connection with saturable reactors controlling the signalling currents of radio transmitters.

Situations arise in the electrical art in which it is desired to be able to produce current waves of trapezoidal wave form, i. e. a wave in which current rises nearly linearly with time from a trough value to a crest value, then flows unaltered at this crest value for a time, then drops nearly linearly with time to the trough value again and continues for some time at that value. The curve A in Fig. 1 herein a wave of this type. One use for such a current with which I will illustrate my present invention is asa saturating current in the control winding of a saturable reactor incorporated in the antenna of a radio transmitter of large power keyed with teletype signals.

In general, the circuits traversed by the trapezoidal currents will comprise capacitance, inductance and resistance, but often the capacitance may be neglected. Where currents of large value are carried, energy loss usually makes it desirable that the resistance r be of the minimum value inseparable from practical windings and leads, and the inductance L may be set at some rather substantial value by desired operating conditions. Thus, the ratio may be required to be kept below some prescribed value; such is in fact the case with the saturating winding here described.

One presently known way of causing the current rise, such as that at the leading edge of the trapezoidal wave, is to suddenly switch a constant direct current voltage E into series with the Lr circuit. For small intervals of time At thereafter, the rate of current rise can be shown to be constant; i. e.

I IT=%A where I is the crest Value and I the trough value of the current wave; this formula applies approximately as long as r(I I is negligible compared with E The constant crest value I represented by the horizontal crest value of the current wave is, of course, given by where E is a direct current voltage impressed on the circuit. If B were the same as E the current would, strictly speaking, reach that value only after infinite time; but even in cases where the effect of r on current value during the rise time AI is negligible compared with that of L so that the rise time required for the current to increase from the trough value I to I would be The time in seconds required to produce a required current rise thus could not be less than for the leading edge of the trapezoidal wave, and. the foregoing equation shows it to equal for the circuit.

It is thus evident that where the trapezoidal current wave is required to have a rate of fractional current rise greater than the voltage E, applied to the circuit during the rise time must be greater than the voltage required to send the crest current through the circuit resistance.

Since the downward slope of the trailing edge of the trapezoidal current wave is equal to the upward slope S, the impression on the circuit of a negative voltage -E, during the time period of the trailing slope, will evidently return the current to the trough value I A constant voltage E =rI for the duration of the trough will evidently maintain the trough value I until voltage E is impressed again at the outset of the next wave.

The voltage required to be impressed on such a circuit to produce trapezoidal waves thus may be considered to consist of a series of four rectangular pulses E E, -E and E of difierent amplitudes. Probably, the simplest way of producing such a voltage is to provide a grid controlled rectifier (such as rectifier 12 of Fig. 2) which generates the voltages E and E and a pulse transformer having a high voltage direct current source (such as transformer 13 of Fig. 2) switched into its primary during the current-rise and current-drop intervals to generate the voltage pulses E and E In the saturated reactor by which I exemplify the foregoing principles, the current rises from a trough value of 25 amps. to a crest value of amps. in 0.002 second in a circuit of inductance L equal to 0.01 henry and resistance r equal to 0.16 ohm.

One object of my invention'is accordingly to make it possible to rapidly change the value of current flowing through a highly inductive circuit of low resistance.

Another object is to provide means for rapidly raising or reducing direct current flow through a circuit having a high ratio of self-inductance to resistance.

Another object is to provide a radio antenna having Patented Sept. 23, 1958 a high Q with means and for abruptly varying its resonant frequency at a minimum expenditure of control power. Another object is to provide a novel arrangement for modulating with signal pulses the output of.a radio transmitter. I

Still another object is to provide an improved arrangement for frequency shift keying of a radio antenna.

Yet another object is to provide an improved system for producing square-top current waves in an inductive circuit.

Other objects of my invention will become apparent upon reading the following description taken in connection with the drawing, in which:

Figure 1 is a graph of certain current and voltage Waves relative to my invention;

Fig. 2 is a schematic diagram of a portion of a circuit for a teletype radio transmitter embodying one form which the principles of my invention may take; and

Fig. 3 is a similar view of a modified form which the Fig. 2 circuit may take.

it is believed that the foregoing discussion has sulficiently described Fig. 1.

Referring in detail to Fig. 2, a teletype or dot-dash code radio transmitter may have an antenna 1, supplied with radio frequency voltage of constant magnitude through an input transformer 2 and having a resonant frequency determined by its capacity to ground (symbolized at 3) and its inductance. A part of its inductance comprises a winding 4 on a saturable magnetic core 5.

When the inductance of winding 4 changes, the resonant ated from antenna 1 then consist of pulses or bursts of one wavelength separated by intervals of a second wavelength. For example, the dots of a telegraph code may comprise bursts of short duration and the dashes be bursts of similar wavelengths but longer duration respectively separated by spaces of a different wavelength. Facsimile signals may consists of of radiated waves of one frequency to reproduce black points on a picture being transmitted, with waves radiated at the second wavelength where the picture is white. The direct current in Winding 6 must accordingly shift between two values which, by altering the saturation of core 5, shift the inductance of winding 4, and hence the resonant frequency of antenna 1, from one to the other of two wavelengths. The winding current 6 must thus be shifted in response to a signalling current controlled by a telegraph key or the scanning of a black-and-white picture or the like. Methods of deriving such signalling currents are Well known and will not be described here. Signalling currents of the type just referred to in general comprise series of substantially rectangular waves of the general form shown at A in Fig. 1 and 7 in Fig. 2, which may be impressed through a shaper 8 on control grids of an oscillation generator 9 generating currents of a wavelength which is high relative to the duration of pulse 7; for the facsimile transmitter which here illustrates my invention, the generator 9 may have a frequency of 1800 cycles per second. During the positive crest period of the pulse 7, generator 9 impresses a higher voltage on the primary of transformer 11, and during the troughs between pulses 7, the voltage of generator 9 falls to a much lower value. The secondary winding of transformer 11 supplies voltage to a bridge-type rectifier 12, which may, for example, be of the well known selenium type, and the output terminals of the latter impresses on the winding 6 rectangular voltage waves which are substantial duplicates of the ill signal pulses 7. Preferably a negative feedback is turnished to oscillation generator 9 by resistor 34.

A pointed out above, the winding 6 has a relatively high self-inductance (around 0.01 henry); and to produce the abrupt shifts in self-inductance of winding 4, which are desired for signalling reasons, it becomes necessary to have the current in winding 6 jump from its trough value to its crest value in the rectangular wave, like 7, in about 0.002 second. The rectangular wave output voltage of rectifier 12 could cause such an abrupt current jump if the resistance of the winding 6 were made high enough; but this would result in a highly undesirable waste of energy (around 320 kilowatts) in continued heating of that resistance. The need for such resistance and energy waste is avoided by the other portions of the Fig. 2 circuit which I will now describe.

The expedient which I employ to speed up the current rise in winding 6 from trough to crest of its nearly rectangular wave form is the introduction, by the secondary winding of a transformer 13, of a positive voltage pulse in series with the output voltage of rectifier 12 during the 0.002 of a second following the arrival of a signal pulse 7; and of a negative voltage pulse at the termination of pulse 7 to quickly drop the current in winding 6 from its crest value to its trough value. The primary winding of transformer 13 has its end terminals connected respectively to the anodes of a pair of push-pull connected electron tubes V3 and V4 whose cathodes are connected to the mid-point of said primary winding and to the positive pole 14 of a direct current source which has its negative pole grounded. The control grids of tubes V3, V4 are interconnected by a resistor having its mid-point connected to their cathodes. The mid-points on each half of the primary winding of transformer 13 are respectively connected to the anodes of a pair of electron tubes V and V having their cathodes grounded.

The output of shaper 8 impresses a pulse 16, which is a sharp-cornered version of pulse 7, on a pair of differentiators 17 and 18, of a type well. known in the electronics art, to produce a pair of waves 19 and 21. The former has a sharp positive spike at the leading edge of wave pulse 7 and a sharp negative spike at its trailing edge; whereas, wave 21 is oppositely poled to have a sharp negative pulse at the beginning end of pulse 7 and a sharp positive spike at its trailing edge. Clippers 22 and 23 of well known form then eliminate the negative spike in each case, so that clipper 23 delivers a sharp positive spike 26 to a multivibrator 27. The multivibrator 25 is designed in a well known ways to deliver a rectangular pulse 28 of length about equal tothe desired rise time of the current wave of saturating winding 6 at a time coincident with the leading end of pulse 7; and similarly, multivibrator 27 delivers a rectangular pulse 29, of similar duration, coincident in time with the trailing end of pulse 7.

The pulse 28 is also reversed in polarity, e. g. through an amplifier tube 31, and impressed as a negative pulse on the grid of tube V4. Similarly, pulse 29 is reversed, e. g. through amplifier tube 32, and impressed on the grid of tube V3. Pulse 28 is impressed as a positive pulse on the grid of tube V1, and pulse 29 is impressed as a positive pulse on the grid of tube V2.

The mode of operation of the above-described circuit is substantially as follows, starting with the current in winding 6 at its trough value, e. g. 25 amps. The impressing of the positive pulse output of shaper 8 on the control grids of oscillator 9 raises the output voltage of rectifier 12 to a value sufiieient to send amps. through the resistance of the winding 6. At the same time, the positive voltage pulse 28 is applied to the grid of tube V1 to render it conductive, and the voltage of positive terminal 14 is impressed across the inner sector of the upper half of the primary winding of transformer 13. This results in the secondary winding of transformer 13 impressing a voltage pulse for substantially the duration of pulse 28 in the circuit of winding 6, and the secondary winding of transformer 13 is proportioned so that the magnitude of this'pulse sufiices to overcome the selfinductance of winding 6 and raise the current therein to 125 amps. in 0.002 second, the duration of pulse 28.

The voltage impressed through tube V1 on the upper half of the primary winding of transformer 13 induces a positive voltage of about twice that of terminal 14 on the anode of tube V4 and, were it not for the negative voltage pulse impressed by tube 31 on the grid of V4, would result in a short circuit of the system. However, the said negative pulse keeps tube V4 non-conductive throughout the duration of pulse 28. On termination of pulse 28, however, removal of that positive pulse from the grid of tube V1 ends the voltage pulse impressed by the secondary of transformer 13 on winding 6 and the current therein ceases to rise above 125 amps.

While the current is flowing at crest value in the secondary winding of transformer 13, there is, of course, a corresponding current component of opposite magnetic effect flowing in the primary winding through tube V1, and when the positive pulse 28 on the grid of the latter ends, the flow of this current component is opposed by the plate-resistance of tube V1, and this reacts to induce a voltage in the secondary winding of transformer 13. This voltage would tend to reduce the saturating current in winding 6 were it not for the presence of tube V3 which, in absence of voltage pulses impressed on its grid by amplifier 32, permits the current, previously flowing from the upper half of the transformer 13 primary through tube V1, to transfer to tube V3 where its flow opposes only a moderate reducing effect on the maintenance of full crest value of the current saturating winding 6.

At the termination of pulse 7, the grid voltage on oscillator 9 drops to a value just sufficient to overcome the IR drop 'of the trough value (i. e. 25 amps.) of current in winding 6. Likewise, the pulse 29 is impressed in a positive sense on tube V2 rendering it conductive and impressing the voltage of terminal 14 across the inner sector of the lower half of the primary of transformer 13. This impresses through the secondary winding thereof a voltage poled to overcome the opposition preferred by the self-inductance of winding 6 to the decrease of current therein. As will be evident from symmetry considerations, this volt-age pulse impressed on the winding 6 circuit will just suflice to reduce the winding 6 current from 125 amps. to 25 amps. if the pulse 29 has the same duration as pulse 28 did. At the same time impression of pulse 29 in negative sense on the grid of tube V3 prevents current flow in the latter from short-circuiting the system as long as pulse 29 endures.

At the termination of pulse 29, the current in winding 6 has reached its original trough value, and the system has passed through a typical cycle which it traverses on the advent of each pulse, such as pulse 7.

Such reducing effect, as the voltage drops through tubes V3 and V4 reflect through transformer 13 into the current in saturating winding 6, may be minimized by providing a negative feedback around transformer 11 proportional to the current in saturating winding 6 by resistor 34 or other suitable means.

It may be noted that current flows from direct current terminal 14 only during the short intervals when pulse 28 exists, and that during pulse 29 the energy of the direct current magnetic field in core 13 is actually being returned to the electron tube system. Thus, the power consumption of the system is limited to the relatively small 1 R losses in winding 6 and its supply and control \system. This is less than ten percent of that encountered when circuit resistance is, as in certain prior art systems, slied on to effect rapidity of rise and fall in the winding rrent wave form.

In Fig. 3, I show a modified form for the portion of Fig. 2 circuit which is surrounded by a dash-and-dot line and which efiects a still further reduction in the power expenditure of the system. In this circuit, I insert between the common cathode of tubes V3, V4 and the midpoint of the primary winding of transformer 13 a direct current voltage source 35 which is suflicient in value to overcome the voltage drop through tube V3 due to one half the current which flows in that primary when the current in saturating winding 6 is at its crest value (i. e. amps. in the exemplary circuit here described). The grids of tubes V3 and V4 are interconnected through a resistor 36 having its mid-point connected to their cathode. The grids of tubes V3 and V4 are also respectively connected to the anodes of a pair of electron tubes 37 and 38, the cathodes of which are connected to ground through a resistor 39. The grids of tubes37 and 38 are connected to a flip-flop multivibrator 41 of conventional type, the two sides of which are triggered by the pulses 28 and 29. Otherwise, the Fig. 3 circuit is like that of Fig. 2, except that the single rectangle 50 represents the content of rectangles 8, 17, 18, 22 and 23 of Fig. 1.

The mode of operation of the Fig. 3 circuit is as follows. When the current in saturating winding 6 is at its trough value, the voltage impressed by the flip-flop multivibrator 41 on the grid of tube 38 has left the latter conducting and tube 37 non-conducting; hence, tube V4 has such a negative grid voltage that it is cut off, but tube V3 is conducting current which magnetizes the core of transformer 13 in the same sense as does the trough value of the current supplied from rectifier 12. When a positive pulse 28 is generated at the beginning of an input pulse 7, the multivibrator 41 is triggered and flops to cut off tube V3 and start current flowing in tube V4 which has the opposite magnetic effect on transformer 13 to that of the current on saturating winding 6. During pulse 28 while the current through saturating winding 6 is rising from its trough to its crest value, the current is decreasing in tube V3 and rising in tube V4 by an amount just sufficient to neutralize the effect, on the core flux in transformer 13, of this rise in the value of the current flowing in its secondary. From symmetry considerations, it is evident that if tubes 37 and 38 are similar, the current flow through tube V3 during the trough period of current in saturating winding 6 should equal the current flow through tube V4 during the crest period of that current; in short, the voltage of source 35 should suflice to send a current equal to half the difference of said crest and trough values (referred to the transformer 13 primary) through the plate-cathode circuit of tube 37. In the present example, that voltage should sufiice to send 12525 am 5 P 2 circuit, considerable economies are attained by changing to the Fig. 3 arrangement. With the same values for pulse 28, the rise time from trough to crest is cut in half I and the mean pulse power required from transformer 13 is cut to one quarter of the value needed in the Fig. 2 circuit. Furthermore, it is possible to dispense with the negative feedback circuit embodying resistor 34 in Fig. 2, if desired, with a consequent large saving in power through more efficient operation of generator 9, and reduction in size of circuit components generally.

Moreover, limitations inherent in negative feedback of amplifiers prevent the maintenance by that-agency of complete constancy of saturating winding current during the trough period, so that the'Fig. 3 arrangement produces a superior wave form in 'that current to anything practically attainable with the Fig. 2 circuit.

1 claim as my invention:

1. In combination with an inductive device having a control Winding thereon and a sourceof signal pulses for controlling the application of voltage to said controlwinding, a transformer having primary and secondary windings, first and second voltage sources, means connecting said first voltage source in series with the secondary winding of said transformer and the control'winding of said inductive device, means responsive to said source'of signal pulses for periodically raising the output voltage of said first voltage source, further means responsive to said source of signal pulses forproducing first and second control pulses, and means responsive to said control pulses for switching said second source of voltage in series with the primary winding of said transformer with one polarity for the duration of said firstcontrol pulse and with the opposite polarity for the duration ofsaidsecondcontrol pulse.

2. In combination with an inductive device having a control winding thereon and ;,a source of signal-pulses, a transformer having primary and secondary -Windings,'first and second voltage sources, means connecting said first voltage source in series -with the secondary winding of said transformer and the control winding ofsaid inductive device, means responsive to said source of signal pulses for producing first and second control pulses, and means responsive to said control pulses for switching said second source of voltage in'series with the primary winding of said transformer with one'polarity for the duration of said first control pulse and with the opposite polarit for the duration of said second control pulse.

3. In combination with an inductive device having a control winding thereon and a source of signal pulses,.-a transformer having primary, andsecondary windings, first and second voltage sources,-means connecting said first voltage source in series with the secondary Winding of said transformer and the. control winding of said inductive device, means responsive to said source of signal pulses for producing first and second control pulses, the leading edges of said first and second control pulses coinciding respectively with the leading and trailing edges of said signal pulses, and means responsive to said control pulses for switching said second source of voltage in series with the primary winding of said transformer with one polarity for the duration of said first control pulse and with the opposite polarity for the duration of said second control pulse.

4. In combination with an inductive device having a control winding thereon and a source of signal pulses for controlling the application of voltage to said control winding, a transformer having primary and secondary windings, first and second voltage sources, means connecting said first voltage source in series with the secondary winding of said transformer and the control winding of said inductive device, means responsive to said source of signal pulses for producing first and second control pulses, and means responsive to said control pulses for switching said second source of voltage in series with the primary winding of said transformer with one polarity for the duration of said first control pulse and with the opposite polarity for the duration of said second control pulse, said means for switching including a pair of electron valves having control electrodes and connected to respectively provide oppositely-conducting unidirectional current shunt paths for said primary winding, and means for applying said first control pulse to the control electrode of one of said valves and said second control pulse to the control electrode of the other of said valves.

References Cited in the file of this patent UNITED STATES PATENTS 1,901,929 Peterson Mar. 21, 1933 2,382,615 Donley Aug. 14, 1945 2,440,320 Young, Jr Apr. 27, 1948 2,426,225 Krause Aug. 26, 1947 OTHER REFERENCES The Radio Amateurs Handbook, 1946 ed., pages 71-72. 

